1) Field of the Invention
The present invention relates to an optical waveguide, a fabrication method therefor and an optical waveguide device to be applied to various optical transmission systems, such as, for example, a wavelength division multiplexing (WDM) optical transmission system in the field of optical communication.
2) Description of the Related Art
In recent years, construction of a wavelength division multiplexing (WDM) optical communication network (WDM optical transmission system) is proceeding in order to implement a photonic network which can cope with an explosive increase of the data traffic caused by popularization of the Internet and so forth.
In the WDM optical transmission system, in order to reduce the cost, it is effective to apply a technique of a planar lightwave circuit (PLC) which can integrate functions of several optical devices and electronic devices using an optical waveguide to integrate various functions. Further, it is demanded to easily implement minimization and high integration of a PLC device wherein such various functions are integrated.
Here, a silica glass-type buried optical waveguide used in the WDM optical transmission system is described with reference to FIG. 18.
As shown in FIG. 18, the silica glass-type buried optical waveguide includes a lower clad layer 111, an upper clad layer 113 and a core layer 112 encircled by the lower and upper clad layers 111 and 113, all formed on an Si substrate 110.
Such a silica glass-type buried optical waveguide as just described is fabricated in the following manner.
First, silica glass (hereinafter described more particularly) which is a material for the lower clad layer 111 and the core layer 112 is deposited in order, and annealing process is performed to convert the silica glass into transparent glass to form the lower clad layer 111 and the core layer 112 in order on the Si substrate 110.
Then, a mask pattern is formed, for example, by photolithography, and thereafter, dry etching according to the reactive ion etching (RIE) method is performed to remove unnecessary part of the core layer 112 to form a striped core layer (waveguide core) 112 having a desired pattern (waveguide pattern).
Then, silica glass (hereinafter described more particularly) which is a material for the upper clad layer 113 is deposited on the lower clad layer 111 and the core layer 112, and annealing process is performed to convert the silica glass into transparent glass to produce an buried optical waveguide wherein the core layer 112 is encircled by and buried within the lower clad layer 111 and the upper clad layer 113.
Here, in order to reduce the difference between the thermal expansion coefficient of the clad layers 111 and 113 and the thermal expansion coefficient of the Si substrate 110 to suppress birefringence arising from the thermal stress, usually the clad layers 111 and 113 are formed typically using BPSG (borophospho-silicate glass) as a material.
Further, a BPSG film which forms the clad layers 111 and 113 is formed using at least one or more of, for example, tetraethoxysilane (TEOS; Si(OC2H5)4), tetramethoxysilane (TMOS; Si(OCH3)4), triethylphosphate (TEOP; PO(OC2H5)3), trimethylphosphate (TMOP; PO(OCH3)3), triethylborate (TEB; B(OC2H5)3) and trimethylborate (TMB; B(OCH3)3) in combination as an organic source and forming a film of the material having a thickness ranging, for example, from 15 μm to 20 μm. For example, the triethylborate (TEB) is a compound having such a structure as shown in FIG. 19.
Further, it is common to use a material having a refractive index higher than that of the borophospho silicate glass (BPSG) used as a material for the clad layers 111 and 113, such as, for example, a GPSG (germanophospho silicate glass) for the core layer 112.
The GPSG film which forms the core layer 112 is formed using at least one or more of, for example, TEOS, TMOS, TEOP, TMOP, tetraethoxygermanium (TEG; Ge(OC2H5)4) and tetramethoxygermanium (TMG; Ge(OCH3)4) in combination as an organic source and forming a film having a thickness of, for example, 10 μm or less.
The material used as an organic source for forming the clad layers 111 and 113 and core layer 112 of such a general optical waveguide is generically called alkoxy-type compound.
It is to be noted that a technique regarding an buried optical waveguide is disclosed, for example, in Japanese Patent Laid-Open No. 63-124006, No. 5-157925, No. 5-100123, No. 5-127032, No. 8-179144, No. 2001-51143 and No. 2001-183538.
Incidentally, in a buried optical waveguide used for a WDM optical transmission system, various functions must be integrated. Therefore, since the striped core layer 112 has a fine pattern (core pattern) formed by means of dry etching according to the RIE method as described above, a fine groove D appears between striped core layers 112 adjacent to each other.
Therefore, when the upper clad layer 113 is formed, silica glass (for example, BPSG) used as a material for the upper clad layer 113 is deposited such that the fine groove D is buried within the upper clad layer 113 without a void (burying failure), and thereafter, annealing process is performed for the silica glass so as to reflow. Consequently, the silica glass (for example, BPSG) used as a material for the upper clad layer 113 is filled in the groove D.
However, if the lower clad layer 111 is formed using an alkoxy-type compound as an organic source as described above, then the lower clad layer 111 is softened upon reflow. As a result, the core layer (waveguide core) 112 formed on the lower clad layer 111 may sink as seen in FIG. 18.
FIG. 20 is a composition chart illustrating relationships of cracks and air bubbles to compositions (wt %) of boron (B) and phosphorus (P) where the clad layers (BPSG films) are formed on the Si substrate (silicon substrate) using an alkoxy-type compound as an organic source.
In FIG. 20, each region of slanting lines indicates a film formation defect appearing composition region wherein cracks or air bubbles (film formation defects) are liable to be formed, and the other region indicates a free composition region wherein cracks or air bubbles are nor formed.
When the compositions (wt %) of boron (B) and phosphorus (P) included in a clad layer (for which an alkoxy-type compound is used) is determined, they must be determined such that cracks or air bubbles are not formed. However, since the free composition region is small as seen in FIG. 20, the determination is constrained in this regard.
Further, where a clad layer is formed on a Si substrate, if there is a difference between the coefficients of thermal expansion of the Si substrate and the clad layer, then stress (internal stress) occurs in the clad layer.
Here, in FIG. 20, a broken line indicates an equal stress line where the stress is zero (stress=0). For example, if the composition is adjusted such that the sum of the B composition and the P composition is approximately 11 wt % (B composition+P composition=11 wt %), then the stress is zero (stress=0). It is to be noted that, in accordance with the magnitude of the stress, the equal stress lines for different stresses can be represented as parallel lines to the equal stress line where the stress is zero.
In FIG. 20, on the left side (lower side) of the broken line, the stress is inclined to increase in a compression direction in accordance with a decrease of the boron composition (B composition) and the phosphorus composition (P composition) (conversely speaking, the stress is inclined to decrease in a compression direction in accordance with an increase of the B composition and the P composition). Meanwhile, on the right side (upper side) of the broken line, the stress is inclined to increase in a tension direction in accordance with an increase of the boron composition (B composition) and the phosphorus composition (P composition) (conversely speaking, the stress is inclined to decrease in the tension direction in accordance with a decrease of the B composition and P composition). In short, in FIG. 20, the stress is lowest on the equal stress line indicated by the broken line, but the stress increases as the distance from the equal stress line increases.
Here, that the stress is zero signifies that the thermal expansion coefficient of the clad layer and the thermal expansion coefficient of the Si substrate are equal to each other (the difference in thermal expansion coefficient is zero: thermal expansion coefficient difference=0). Therefore, where the stress is zero, the equal stress line can be regarded as an equal thermal expansion coefficient line.
In FIG. 20, on the left side (lower side) of the broken line, the thermal expansion coefficient is inclined to decrease in accordance with a decrease of the boron composition (B composition) and the phosphorus composition (P composition), but on the right side (upper side) of the broken line, the thermal expansion coefficient is inclined to increase in accordance with an increase of the boron composition (B composition) and the phosphorus composition (P composition). Therefore, in FIG. 20, on the left side (lower side) of the broken line, the difference of the coefficient in thermal expansion (dclad−dSi) between the Si substrate and the clad layer is inclined to increase in accordance with a decrease of the B composition and the P composition (that is, a negative value increases), but on the right side (upper side) of the broken line, the difference in thermal expansion coefficient (dclad−dSi) between the Si substrate and the clad layer is inclined to increase in accordance with an increase of the boron composition (B composition) and the phosphorus composition (P composition) (that is, a positive value increases). In short, the thermal expansion coefficient of the clad layer is nearest (equal) to that of the Si substrate 110 on the equal thermal expansion coefficient line indicated by the broken line in FIG. 20, but the difference between the coefficient of the Si substrate 110 and the coefficient of the clad layer increases as the distance from the equal thermal expansion coefficient line increases.
Further, as seen in FIG. 20, the P composition has a dominant influence on the refractive index. In particular, the refractive index increases as the proportion of the P composition increases [that is, the proportion of the P composition (wt %) in the BPSG composition increases], but the refractive index decreases as the proportion of the P composition decreases [that is, the proportion of the P composition (wt %) in the BPSG composition decreases]. It is to be noted that, in FIG. 20, a long dashed short dashed line indicates an equal refractive index line of a refractive index equal to a target refractive index for the upper clad layer 113.
Since the stress of the upper clad layer 113 which covers the side face of the core layer 112 has a dominant influence on the birefringence of the core layer 112, the composition of the upper clad layer 113 is preferably selected such that the stress thereof is zero (stress=0) (the difference between the thermal expansion coefficient of the Si substrate 110 and the thermal expansion coefficient of the upper clad layer 113 is zero; linear expansion coefficient difference=0).
Therefore, generally the composition of the upper clad layer 113 is determined such that the P composition is adjusted so that the upper clad layer 113 has a desired target refractive index and the stress is zero (stress=0). In particular, for example, the composition of the upper clad layer 113 is determined such that it is equal to a proportion (wt %) of the B composition and the P composition at a point at which the equal refractive index line indicated by the long dashed short dashed line in FIG. 20 and the equal stress line indicated by the broken line in FIG. 20 where the stress is zero cross each other.
Meanwhile, the composition of the lower clad layer 111 is usually determined such that it has a substantially equal P composition to that of the upper clad layer 113 so that the refractive index of the lower clad layer 111 is equal to the refractive index of the upper clad layer 113. In short, the composition of the lower clad layer 111 is determined such that it falls, for example, on the equal stress line indicated by the long dashed short dashed line in FIG. 20.
In this instance, if the difference in proportion (wt %) of the B composition between the lower clad layer 111 and the upper clad layer 113 is decreased (if the difference of the B composition is insufficient) so that the B composition and the P composition of the lower clad layer 111 may fall in the free composition region (region other than the regions of slanting lines in FIG. 20) as seen in FIG. 20 in order that cracks or air bubbles may not be formed, then a sink of the core layer 112 appears because the melting point of the BPSG depends much upon the B composition. On the other hand, if a sufficient difference of the B composition is assured so that no sink of the core layer 102 may appear, then cracks will be formed.
In this manner, it is difficult to determine the composition (B composition and P composition) of the lower clad layer 111 so that a sink may not appear in the core layer 112 while preventing formation of cracks or air bubbles.